Here's my situation:
I have a 100 year old, 2300 sq ft (roughly 1300 down 1000 up), two-story home with 10 foot ceilings on the first floor. Since we've lived here (about 2 years) the upstairs is relatively comfortable in the winter (due to heat rising) but is very warm in the summer.

The house has an older 90-percent efficient Trane furnace in the cellar, and has a single 8x14" duct supplying air to five registers upstairs.
There are two return airs in the entire house, both of which are in the floor on the first level (one below the thermostat, and the other about 15' away (albeit around a corner) near the stairs to the upstairs).

I have a location where I could install a 12"x12" chase (maybe bigger, though it's hard to tell exactly) from the cellar to the second floor.

I've had one HVAC company tell me I need to install an air-handler in my attic, and another suggest a zoned system. Money is tight, and this furnace will be failing soon. I can't afford a big change (the options the aforementioned HVAC companies gave were seemingly out of my budget), so I either need to find an inexpensive solution, or just learn to be content with a window air-conditioner in my second floor.

It seems like my options are:
A) A larger, or second, upstairs supply run
B) An upstairs return air
C) A more powerful furnace (though I've been told this one is rated for pretty good pressure)
D) Booster fan(s) in conjunction with one of the above

My question is, what is the best bang for my buck?